Use of a plurality of through-flow dryers to dry a web of textile or paper goods is known. First among these dryers are the screen or sieve drum dryers according to DE-OS 16 04 784, as well as the belt dryers according to DE 29 50 295 A1. It is known to determine the temperature of the ambient air flowing over the goods to be dried by means of thermocouples located in the dryer and to use these measured values to regulate the heating assemblies, taking the incoming air into account in the heating power (DE-OS 16 04 784). It is also known to regulate the fan of a screen drum unit that accelerates the processing air, specifically as a function of the pressure differential between the outside of the screen drum and after the air has passed through the web (DE-OS 16 35 357).
However, the method according to the present invention involves regulating the moisture content of a web to be dried with through ventilation. For this purpose, according to the ITS publication "Farberei/Druckerei/Ausrustung" 3/1967, pages 14-26, especially page 26, it is possible to measure the residual moisture content of a web with electrical auxiliary devices. Measuring devices of this kind however only operate up to hygroscopic equilibrium of the respective goods; they are also inexact and therefore operate unsatisfactorily. To avoid these problems, it is possible to overdry a web and then moisten it again to the desired moisture content. This procedure is uneconomical, however.
A device is known with which the moisture content is measured over the width of the web at the outlet from the dryer, and the exhaust air flap of the dryer is regulated by this measured value. During this measurement process, an infrared measuring head moves transversely across the web. This moisture measurement provides good results, but it is too costly in terms of the cost of the apparatus and hence it is too expensive.